1. Technical Field
The present disclosure relates to backlight control circuits, and more particularly to a backlight control circuit including a duty ratio determining unit, and to a method for controlling lighting of a lamp using the backlight control circuit.
2. General Background
Liquid crystal displays are commonly used as display devices for compact electronic apparatuses because they provide good image quality and because they are very thin. A liquid crystal in a liquid crystal display does not emit any light itself. The liquid crystal requires a light source so as to be able to clearly and sharply display text and images. Therefore, a typical liquid crystal display requires an accompanying backlight module. If a cold cathode fluorescent lamp (CCFL) is used in a backlight module, the backlight module generally includes a backlight control circuit. The backlight control circuit is configured for converting a direct current voltage to an alternating current voltage to drive the CCFL.
Referring to FIG. 7, one such backlight control circuit 100 includes a lamp driving circuit 110, a transformer 120, a lamp 130, and a feedback circuit 140. The lamp driving circuit 110 and the transformer 120 constitute an inverter for providing an alternating voltage for driving the lamp 130. The lamp driving circuit 110 is configured for adjusting the alternating voltages provided to the lamp 130 according to a real-time brightness of the lamp 130. The feedback circuit 140 is electrically connected between the lamp driving circuit 110 and the lamp 130. The lamp 130 can, for example, be a cold cathode fluorescent lamp (CCFL).
The lamp driving circuit 110 includes a brightness determining unit 112, a duty ratio adjusting unit 114, and an output unit 118. The brightness determining unit 112 is electrically connected to the duty ratio adjusting unit 114 and the output unit 118, respectively. The duty ratio adjusting unit 114 is further connected to the output unit 118. The transformer 120 includes a primary coil 122 and a secondary coil 124. The primary coil 122 is electrically coupled to the output unit 118 of the lamp driving circuit 110. One terminal of the secondary coil 124 is connected to ground via the lamp 130, and another terminal of the secondary coil 124 is connected to ground via the feedback circuit 140. The feedback circuit 140 is further connected to the brightness determining unit 112.
An exemplary method for controlling lighting of the lamp 130 using the backlight control circuit 100 is as follows. The output unit 118 of the lamp driving circuit 110 outputs two pulse signals DR1 and DR2 to two terminals of the primary coil 122 of the transformer 120, respectively. Referring to FIG. 8, a waveform diagram of the two pulse signals is shown. The two pulse signals DR1 and DR2 have opposite phases and the same duty ratio. The two pulse signals induce the primary coil 122 to generate a current having an alternating direction, thereby inducing the second coil 124 to generate an alternating voltage for driving the lamp 130.
The feedback circuit 140 samples the current in the lamp 130, generates a brightness signal according to the sampling current, and outputs the brightness signal to the brightness determining unit 112. The brightness signal can be a voltage signal.
The brightness determining unit 112 has a reference brightness. The brightness determining unit 112 receives the brightness signal from the feedback circuit 140, compares values of the brightness signal and the reference brightness, and outputs a control signal according to the comparison result.
When the value of the brightness signal is equal to or greater than the value of reference brightness, the brightness determining unit 112 outputs a hold signal to the output unit 118, thus the output unit 118 keeps the two pulse signals invariant according to the hold signal.
When the value of the brightness signal is less than the value of the reference brightness, the brightness determining unit 112 outputs a trigger signal to the duty ratio adjusting unit 114. Thus, the duty ratio adjusting unit 114 outputs a duty ratio adjusting signal to the output unit 118. The output unit 118 adjusts the duty ratio of the two pulse signals upward, thereby increasing the current in the lamp 130 and therefore improving the brightness of the lamp 130.
The reference brightness is set according to a size of the lamp 130 and characters of the backlight control circuit 100. Because different backlight control circuits have different characters, it is possible that the brightness of the lamp 130 can not reach the reference brightness. In general, if the brightness of the lamp 130 is always less than the reference brightness, the duty ratio adjusting unit 114 continuously outputs duty ratio adjusting signals to the output unit 118 such that the output unit 118 continuously increases the duty ratio of the pulse signals. However, when the duty ratio of the pulse signals is greater than an upper range value (50%), the brightness of the lamp 130 can not reach the reference brightness yet, and the backlight control circuit 100 may be switched off because automatic protection is started. Therefore, the backlight control circuit 100 has a low reliability.
Therefore, a new backlight control circuit that can overcome the above-described problems is desired. What is also desired is a method for controlling lighting of a lamp using such a backlight control circuit.